Scratchbuilding the Dove part 12

But, The Cockpit Master Has To Be Hollow!  Early on, during the methodology phase of this project, I decided to produce two hollow versions of the cockpit/access hatch. One of these, the 'basic' cockpit-access hatch master would simply indicate the deep-set cockpit windows and external air-scoops positioned either side of the cockpit structure. This basic cockpit master would eventually be bonded atop the hull halve master. Making the cockpit a hollow structure would permit me to work out the angle and depth to the window. But, before bonding this item permanently to the hull master I would use it to produce a second-generation (updated with the scoops and inset window frames) cockpit-access hatch tool. From that tool I would pull a second GRP (glass reinforced plastic) thin walled cockpit piece. That piece would have its access hatch removed and both the hatch and cockpit would be extensively detailed on the inside to represent the interiors of these items as seen in the movie.

With that end game in mind - two types of thin walled GRP cockpit-access hatch masters, one to be integrated with the hull, the other as a detail option - I set about building the tool from which to produce the first thin walled GRP cockpit-access hatch master.

MAKING A COCKPIT TOOL

To fabricate hollow GRP cockpit masters I set about the task of producing a hollow shell copy of the cockpit and access hatch piece by making a hybrid tool (mold). This tool consists of two elements: a hard-shell fiberglass/epoxy resin GRP outer portion, the 'mother mold'. And an interior element, the 'glove mold', made from flexible RTV silicon rubber.

I set about the task of producing a hollow shell copy of the cockpit and access hatch piece by making a hybrid tool (mold). First, the master was mounted on a flat moldboard, protected with aluminum foil, covered with a layer of clay, and a GRP mother mold shell built up over the clay.

I direct your attention to the picture showing the solid modeling foam based cockpit master mounted on a mold board, some clay to the left, and in the background part A and part B of the 'West System' epoxy resin I would use for mother mold fabrication.

The Gougeon Brothers, Inc. manufactures the West System epoxy resin I favor. Their phone number is (517) 684-7286. Ask for their extensive line of 'how to' brochures, and product catalog. The resin used here is their #105-B Epoxy Resin, and #105-A Fast Epoxy Hardener. Most well stocked marine supply outlets and plastic supply houses carry the West System line. Insure that you also purchase the West System 5:1 set of dispensing pumps - making mixing the resin a trouble free process.

Creation of this hybrid tool starts with fabrication of the hard-shell mother mold. First, three coats of mold release wax were rubbed onto the surface of the moldboard to inhibit adhesion of the laminating resin used to bind the fiberglass that gives strength and rigidity to the mother mold element.

C.A. Nash & Son, Inc. Distributors, carry the absolute best mold release wax I have used. The name of the stuff is Bowling Alley Wax. The address is, Norfolk, VA 23508. Sorry, no phone number. You're going to have to look around real hard to find this stuff, but it is worth the effort.

I then mounted the cockpit master onto the moldboard - this accomplished by inserting clay between the bottom of the master and the surface of the moldboard. The clay was thick enough to stand the master about an eighth-inch off the surface of the moldboard. The clay was worked around the outer perimeter of the master with a slight undercut. I then took a piece of kitchen aluminum foil and wrapped it around the entire master. The foil worked to protect the master from the very messy clay.

About the clay: most catalyzed RTV silicon rubbers are inhibited from attaining a complete cure by any sulfur bearing materials it comes into contact with. Make sure that the clay you use is of the oil-base type, not 'natural' sculpting clay. In this application, sticking the master to the moldboard, I'm taking advantage of the 'stickiness' of this type clay to not only stand the piece off the moldboard, but to actually adhere it in place - very versatile stuff.

I use the oil-based Prang Modeling Clay #00749 (a cream color, but any other color they offer will do an effective job in this application). Distributed by the Dixon Ticonderoga Co., Maitland, FL 32751. Sorry, no phone number. Check out the art supply and children's play supply houses for this stuff.

If in doubt about the materials that will come into contact with the RTV you 're using, perform this test: Set samples of the material out on a table and pour a small amount of catalyzed RTV over them. Those items that prevented a complete cure must either be replaced with something else or will have to be over-coated by a suitable primer barrier before being used to create a rubber tool.

An half-inch thick layer of modeling clay was built up over the cockpit and access hatch master. The clay is a stand-in for the later produced RTV rubber glove mold that will capture the exact shape of the master. Note the circular clay dam built around the work - this to prevent overflow of laminating resin during fabrication of the mother mold.

EXPERIMENT!

Note the photo of an oval circle surrounding a blob of clay. We're looking at the cockpit master already encased in a half-inch thick layer of modeling clay. The 'skin' of clay here produces the annular space between the inner surface of the mother mold and outer surface of the master - this space later to be occupied by the rubber glove mold element of this hybrid tool.

The circular clay dam built around the work is there to prevent any overflow of laminating resin from spilling over and making a mess of my workbench.
 

I mixed the epoxy laminating resin with its hardener and added a thickening agent. This 'gel-coat' was brushed over the clay clad master and the run-off permitted to collect on the mold board between master and clay dam. Just in case things got out of control I placed the moldboard over a sheet of aluminum foil.

Build up of the mother mold started by mixing a quantity of epoxy laminating resin with its hardener and then adding a thickening agent. The thickening agent works to help hold the resin from freely running off of the vertical surfaces. This 'gel-coat' was brushed over the clay-clad master. The run-off permitted to collect on the moldboard between master and clay dam. Just in case things got out of control (what can go wrong WILL go wrong!) I placed the moldboard over a sheet of aluminum foil.

The thickening agent of choice is Gougeon Brothers, #406 Colloidal Silica Adhesive Filler. You can get this stuff at the same source you get the other West System products.

Completing the fabrication of the mother mold was the lay-up of four laminations of seven-ounce fiberglass cloth. The mother mold was left to cure overnight before popping it off the moldboard.

Completing the fabrication of the mother mold was the lay-up of four laminations of seven-ounce (this stuff is classed as its weight per square yard) fiberglass cloth. For these kinds of chores I use cloth with weights between two and ten ounces. The more radical of curvature the subject, the lighter of weight must be the glass selected in order for it to posses the flexibility needed to conform, without wrinkles, to the surface of the work. Once the layers of glass had been laid down and wetted with resin, the mother mold was left to cure overnight.

In the morning I drilled three holes, equally distanced around the perimeter of the mother mold flange, these holes extending well into the moldboard. These holes to accept three wood screws. These fasteners would assure that the mother mold would be positioned over the cockpit master with perfect registration as I got on with the RTV rubber glove mold fabrication later.

I slipped a putty knife blade between the flange and the surface of the moldboard and applied a prying force. With little effort, the entire foil/clay/mother mold unit popped off the master and moldboard. As I already mentioned, the foil was wrapped around the master to simplify the clean up. I then scraped away the clay dam from the perimeter of the mother mold and scooping out the clay sheath from the inner cavity of the mother mold.

After the GRP mother mold had cured hard I slipped a putty knife blade between the flange and the surface of the moldboard. With little effort, the entire foil/clay/mother mold unit popped off the master. I then set about the task of scraping away the clay damn from the perimeter of the mother mold and scooping out the clay standoff material from the inner cavity of the mother mold.

The final shot of this installment shows the inverted, removed mother mold.

On a power sander I ground down the mother mold flange to an even outline. After scraping out as much clay as I could with a Popsicle stick I removed the remaining clay by soaking the mother mold in lacquer as I scrubbed out the remaining clay with a stiff brush.

Next I'll cover how the RTV silicon rubber glove mold element of this hybrid tool is created.